Thermal Rearrangement of Ethynylarenes to Cyclopentafused Polycyclic Aromatic Hydrocarbons: An Electronic Structure Study

Abstract

BLYP/6-311G** electronic structure calculations are employed to elucidate the reaction mechanism of the thermal rearrangement of ethynylarenes to cyclopentafused polycyclic aromatic hydrocarbons (CP-PAHs). Out of two potentially possible unimolecular rearrangement pathways, a one-step process involving hydrogen migration immediately followed by ring closure is clearly favored thanks to a significantly lower energy barrier. Contrary to common belief, neither 2-(1-naphthyl)ethylidene carbene nor its annelated analogs are genuine chemical species. Consequently, these carbenes do not constitute intermediates pertinent to this reaction mechanism, which has the predicted activation energy of 55−62 kcal/mol depending on the extent of strain in the parent ethynylarene molecule. The present study conclusively rules out free-radical mechanisms that commence with hydrogen abstraction. However, in the case of strainless substrates and higher reaction pressures, the pathway involving initial hydrogen addition might become a viable competitor to the one-step unimolecular rearrangement.